One-dimensional metal nanostructures

ABSTRACT

Tin powder is heated in a flowing stream of an inert gas, such as argon, containing a small concentration of carbon-containing gas, at a temperature to produce metal vapor. The tin deposits as liquid on a substrate, and reacts with the carbon-containing gas to form carbon nanotubes in the liquid tin. Upon cooling and solidification, a composite of tin nanowires bearing coatings of carbon nanotubes is formed.

This application is a division of Ser. No. 11/850,860 filed Sep. 6,2007. This application claims priority based on provisional application60/824,910, titled “One Dimensional Metal and Metal OxideNanostructures,” filed Sep. 8, 2006, which is incorporated herein byreference.

TECHNICAL FIELD

This disclosure pertains to one-dimensional metal and metal oxidenanostructures (for example, structures of tin, tin oxide, titaniumoxide, and tungsten oxide) with variable compositions and morphologies.This disclosure also pertains to methods for making and dopingnanostructures in forms such as wires, rods, needles, and flowers.

BACKGROUND OF THE INVENTION

Nanostructured materials (i.e., structures with at least one dimensionin the range of 1-100 nm) have attracted steadily growing interest dueto their unique, properties and potential applications complementary tothree-dimensional bulk materials. Dimensionality plays a critical rolein determining the properties of materials due to, for example, thedifferent ways that electrons interact in three-dimensional (3D),two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D)structures. Compared with 0D nanostructures (so-called quantum dots ornano-particles) and 2D nanostructures (thin films), 1D nanostructures(including carbon nanotubes (CNTs) and nanowires (NWs)) are ideal asmodel systems for investigating the dependence of electronic transport,optical, and mechanical properties on size confinement anddimensionality as well as for various potential applications, includingcomposite materials, electrode materials, field emitters,nanoelectronics, and nanoscale sensors.

Nanowires are a class of newer one-dimensional nanomaterials with a highaspect ratio (length-to-diameter typically greater than 10). They haveinterest separately from carbon nanotubes. Nanowires can be made ofvarious compositions of materials in addition to carbon. Nanowires havedemonstrated superior electrical, optical, mechanical and thermalproperties. For example, the ultrahigh-strength of gold nanowires hasrecently been demonstrated. The significant increase in strength is dueto reduced defects in the crystal structure and a smaller number ofgrains crossing the diameter of the nanowires. The broad choice ofvarious crystalline materials and doping methods makes the properties(e.g. electrical) of nanowires tunable with a high degree of freedom andprecision.

Nanowires consist of a variety of inorganic materials includingelemental semiconductors (Si, Ge, and B), Group III-V semiconductors(GaN, GaAs, GaP, InP, InAs), Group II-VI semiconductors (ZnS, ZnSe, CdS,CdSe), and metal oxides (ZnO, MgO, SiO₂, Al₂O₃, SnO₂, WO₃, TiO₂). Amongthem, metal oxide nanowires have obvious advantages for some specialapplications due to their unique properties such as strong chemicalinteraction with metallic components. This phenomenon is sometimesexplained as strong metal-support interaction. Significant progress hasbeen reported in the use of metal oxide nanowires and nanobelts assensors and in other electronic applications.

Substantially one-dimensional nanostructures (e.g. nanowires, nanorods,and nanobelts having a length much larger than thickness) are a newclass of nanomaterials. Synthesis methods for such nanostructuresusually fall into two categories: vapor-phase deposition orsolution-based crystal growth. While solution-based syntheses generallyoffer better control of processing conditions and more easily achievehigher productivity, vapor deposition often yields higher aspect ratios(for example, length-to-width or length-to-diameter ratios) andexcellent crystallinity due to the higher growth temperatures. However,one of the prominent current challenges is in controlling the synthesisof metal oxide nanostructures in ways that allow variation in theirmorphology. This would permit exploration of different potentialmaterials applications of the nanostructures as their shapes arechanged.

SUMMARY OF THE INVENTION

Metals and metal oxides are formed in substantially one-dimensionalnanostructures having a variety of shapes such as wires, rods, needles,belts, and flowers (wires or rods joined at a center and extending likeradiating petals). Nanowires and rods have a uniform diameter from endto end while nanoneedles have sharp tips. These nanostructures have ahigh aspect ratio (e.g., ratio of length to diameter or thickness of tenor greater) where the smaller dimension is in the range of one to onehundred nanometers. Examples of metals that may be processed bypractices disclosed in this specification include tin, titanium, andtungsten.

In a first embodiment, a powder of the metal is heated in a stream of aninert gas such as argon. Suitable temperatures are often in the range ofabout 700° C. to about 1000° C. The metal powder is heated to a suitabletemperature at which it produces a quantity of vapor (e.g., tin), andthe inert gas contains a small amount of oxygen molecules (for examplein parts per million). Oxygen reacts with the hot metal vapor to formoxidized metal which is solid at the reaction temperature and depositsas one-dimensional nanostructures on a nearby substrate. When the metalproduces little vapor at the temperature of the chamber (e.g., titaniumand tungsten) the one-dimensional nanostructures may be grown directlyon the metal powder.

The oxygen content of the inert gas stream may be low enough such thatnon-stoichiometric metal oxides are formed. For example,oxygen-containing metal compositions such as SnOx, TiOx and WOx may beformed where “x” is not an integer reflecting a common oxide such asSnO, SnO₂, TiO₂, WO₃, etc.

Oxygen for oxidation of the metal powder particles is obtained from oneor more several sources, including: oxygen initially absorbed on thestarting metal material; the controlled addition of oxygen into theflowing inert gas; leakage of air into the inert gas; and/orhumidification of the inert gas to add water molecules, some of whichdissociate to oxygen and hydrogen in the hot reaction medium.

The metal oxide nanostructures may be doped with sulfur, carbon, or thelike by adding the doping element to the inert gas stream flowing overthe vapor-generating metal powder. Sulfur may be vaporized into theinert gas upstream of the metal powder. A hydrocarbon gas such asmethane, ethylene or acetylene may be added to the inert gas upstream ofthe metal powder.

A tubular flow reactor heated by an enclosing electric furnace mayprovide a suitable reaction system. In this embodiment a container forthe powder is paced within the heated zone of the tubular reactor and asuitable substrate for nanostructure growth provided at a suitablelocation near the metal powder. The one-dimensional nanostructure shapetaken by the condensing oxygen-containing metal is found to depend onthe parameters of the reacting system, including the location ofsubstrate material and nature of dopant material if it is employed.

In another embodiment of the invention, composites of carbon nanotubescontaining tin nanowires are formed when a stream of argon containing,for example, about 2% ethylene is passed over tin powder in a reactionvessel at 900° C. The composite of one-dimensional nanostructures may beformed on a substrate such as carbon fiber paper. In this embodiment,the low-melting tin vapor is deposited as liquid droplets on the carbonfiber substrate. The tin droplets catalyze the decomposition of ethyleneand the formation of carbon nanotubes. Upon cooling of the reactor theliquid tin droplets solidify and shrink as one-dimensional tin nanowireswithin the carbon nanotubes.

The resulting one dimensional metal-carbon composite nanostructures andone-dimensional metal-oxygen nanostructures have utility, for example,in electrical/electronic applications and as catalyst supports.

Other objects and advantages of the invention will be apparent fromdescriptions of preferred embodiments which are presented in thefollowing specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a chemical vapor deposition (CVD)reactor device to grow non-doped metal or metal oxide nanostructures ina flowing stream of oxygen-containing argon.

FIG. 1B is a schematic diagram, like FIG. 1A for growing sulfur-dopedmetal or metal oxide nanostructures by evaporating sulfur powder.

FIG. 1C is a schematic diagram, like FIG. 1A, for growing carbon-dopedmetal or metal oxide nanostructures by introducing ethylene gas.

FIG. 2A is a scanning electron microscope (SEM) image of a bare carbonpaper substrate.

FIG. 2B is an SEM image of SnOx nanowires grown on carbon fibers of acarbon paper substrate.

FIG. 2C is an SEM image of SnOx needles grown on a carbon papersubstrate.

FIG. 2D is an SEM image of two generations of SnOx nanostructures grownon carbon paper substrate.

FIG. 2E is an SEM image of SnOx nanobelts in a ceramic boat substrate.

FIG. 3A is an SEM image of carbon-doped SnO_(x) nanostructures grown oncarbon paper substrates under conditions of 900° C., 200 sccm argon gaswith 0.5 sccm of ethylene gas (C₂H₄) for 2-4 hrs.

FIG. 3B is an SEM image of carbon-doped SnO_(x) nanostructures grownlike the sample in FIG. 3A with an ethylene gas flow rate of 1 sccm.

FIG. 3C is an SEM image of carbon-doped SnO_(x) nanostructures grownlike the sample in FIG. 3A with an ethylene gas flow rate of 2 sccm.

FIG. 3D is an SEM image of carbon-doped SnO_(x) nanostructures grownlike the sample in FIG. 3A with an ethylene gas flow rate of 5 sccm.

FIG. 3E is an SEM image of carbon-doped SnO_(x) nanostructures grownlike the sample in FIG. 3A with an ethylene gas flow rate of 7 sccm.

FIG. 3F is an SEM image of carbon-doped SnO_(x) nanostructures grownlike the sample in FIG. 3A with an ethylene gas flow rate of 10 sccm.

FIG. 3G is an SEM image of carbon-doped SnO_(x) nanostructures grownlike the sample in FIG. 3A with an ethylene gas flow rate of 12 sccm.

FIG. 4A is a scanning electron microscopic (SEM) image of carbon-coatedtin nanostructures on a carbon fiber. The nanostructures in FIGS. 4A-4Fwere formed by reaction of tin powder with 900° C., 200 sccm Ar gas and2 sccm ethylene gas for 2 hrs.

FIG. 4B is an SEM image of a carbon-coated tin nanoflower structure on acarbon fiber.

FIG. 4C is an SEM image of tips of carbon-coated tin nanowires.

FIG. 4D is a transmission electron microscope image (TEM) of the wiresof FIG. 4C showing a portion of the tubes near the tips of the wires.

FIG. 4E is a TEM image of an individual tin nanowire showing the carbonnanotube shape near the tip of the wire. The insert in FIG. 4E is an EDXplot confirming the existence of a tin nanowire rather than a SnOxnanowire.

FIG. 4F is a TEM image of an individual tin nanowire showing the tube onthe tip.

FIGS. 5A and 5B are SEM images of un-doped WOx nanowires at 4000-fold(A) and 12,000-fold (B) magnification, respectively.

FIGS. 5C and 5D are SEM images of sulfur-doped WOx nanowires at10,000-fold (C) and 20,000-fold (D) magnification, respectively.

FIGS. 5E and 5F are SEM images of carbon-doped WOx nanowires, each at10,000-fold magnification.

FIG. 6 is a high resolution TEM image of a carbon-doped WOx nanowireshowing amorphous carbon on the surface of the nanowire.

FIG. 7A is a SEM image (5000×) of original titanium powder beforegrowing TiOx nanostructures.

FIG. 7B is a SEM image (2000×) of a few TiOx nanostructures grown inargon on titanium powder particles. The structures were grown by CVD at800-900° C. for 1-4 hrs.

FIGS. 7C and 7D are SEM images at 1,000-fold (C) and 5,000-fold (D)magnification, respectively, of TiOx nanoneedles grown by CVD underargon with water.

FIGS. 7E and 7F are SEM images at 5,000-fold (E) and 10,000-fold (F)magnification, respectively of TiOx nanowall structures grown by CVDunder argon with water and 20 volume percent acetone.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following experimental procedures were followed in the preparationof one-dimensional nanostructures of the oxides of tin (SnO_(x)),titanium (TiO_(x)), and tungsten (WO_(x)), respectively, starting withcommercial powders of tin, titanium, and tungsten as obtained fromAldrich Chemical Company.

Nanostructures, nanowires, nanorods, nanobelts and nanoneedles, of metaloxide SnO_(x), WO_(x), and TiO_(x) were synthesized by a chemical vapordeposition method. Here, the values of x are usually greater than zeroand up to, for example, three or greater, depending on the oxidationstate of the metal. The values are not necessarily integer values and donot necessarily represent stoichiometric compounds. In some instancesnanostructures of the metal were obtained.

The experimental setup (see FIGS. 1A-1C) included a horizontal tubefurnace with an electrically resistance heated zone of about 30 cm inlength. A quartz tube, 60 cm long, was centered along its length andenclosed in the furnace. Provision was made for the controlled flow ofargon gas into one end of the tube (right end in FIGS. 1A, 1B, and 1C)and through the quartz tube to provide an atmosphere with relatively lowoxygen content for the formation and growth of oxygen-containing metalnanostructures.

In a typical procedure, the metal powder (Sn, Ti, or W powder) wasplaced in an alumina boat (labeled in FIG. 1A) and located at themid-point of the quartz tube and tube furnace. High purity argon(99.999%) was flowed through the quartz tube at a rate of 50 sccm(standard cubic centimeters per minute) for 15 min to remove O₂ andother gases from the quartz tube chamber. The argon, initially at anambient temperature, was rapidly heated within the hot tube furnace andpartially enclosed quartz tube. The vapor deposition chamber thusprovided in the quartz tube, and the argon gas flowing through it, wereheated from room temperature to relatively high temperatures (700-1000°C.). Tin is liquid at these temperatures while titanium and tungsten aresolid.

A small amount of oxygen was required to react with the metal powder, orvapor from the hot metal, and slowly produce the respective particulatemetal oxide materials, which were not necessarily stoichiometriccompounds. The metal particles (typically about 99.8% by weight of therespective metal) inherently initially contained a thin adsorbed coatingof oxygen molecules. Additional oxygen was obtained to grow the metaloxide nanostructures from the very small residual oxygen content of theargon and from a low rate of oxygen leakage from the ends of the quartztube. These small oxygen sources were sufficient to slowly oxidize themetal particles and oxygen-containing metal nanostructures were formed.They typically formed on powder particles (e.g., Ti and W) in theceramic boat, on the sides of the ceramic boat, or on another nearbysubstrate provided for nanoscale particle growth (e.g., in the case ofthe liquid tin).

After growing the metal or metal oxide nanostructures for a period oftime (e.g., 1-4 hrs), the furnace was cooled down to room temperature asthe flow of argon was continued.

Carbon paper was used as a substrate for growth of SnOx nanostructures.The commercially-used carbon paper is a class of electrode materials forvarious applications such as fuel cells and sensors. A piece of thecarbon paper was paced on the ceramic boat shown in FIG. 1A.

WOx and TiOx nanostructures, in the form of free-standing powder, couldbe synthesized directly from and on their powder as a result of theirrelatively high melting points and relatively low vapor pressure atreaction conditions.

Water as an Oxygen Source

In order to increase and further control the amount of oxygen during thesynthesis, water was introduced with argon gas during some of the metaloxide synthesis experiments. In the water-assisted oxidation reactions,the argon flow from its storage tank was bubbled through a hot waterbath (80° C.) and then flowed into the quartz tube so that water vaporwas continuously carried into the reaction zone. The water dissociated(partially) in the hot tube to provide additional oxygen for synthesisof the one-dimensional nanostructures. The control of water amount wasmanaged by the temperature of the water and/or the flow rate anddispersion of the bubbled inert gas. The water-assisted oxidationreaction is very effective for the growth of WO_(x) and TiO_(x)nanostructures from their metal powders.

In-Situ Doping of the Metal Oxide Nanostructures with Sulfur

In some experiments the nanostructures were doped with sulfur to modifyelectrical properties of nanostructures and/or the interaction of themetal oxide nanoparticles with subsequently deposited catalystparticles. In-situ sulfur-doping was conducted by placing a container ofsulfur powder upstream of the metal powder (with respect to thedirection of flow of the inert gas) as shown in FIG. 1B. The sulfurreadily vaporized in the heated quartz reactor and was carried in theflowing argon into contact with the oxidized (and oxidizing) metalparticles. The sulfur vapor diffused into the metal oxide nanostructuresas a dopant element.

Sulfur-doped WOx nanostructures were typically obtained by the CVDprocess with the furnace at 760° C. and an argon stream (flow rate, 100sccm, initially bubbled through hot water at 80° C.) passed through theheated quartz tube for 1-4 hrs. A ratio of sulfur to WOx (molar ratioW/S) was about 3:1.

In-Situ Doping of Carbon into Nanostructures.

There is also interest in doping small metal oxide particles with carbonwith the goal of modifying their properties, for example theirelectrical and anti-corrosion properties. Here, carbon-doping wascarried out in three different ways: (i) a hydrocarbon gas (e.g. C₂H₄,CH₄, C₂H₂) was mixed with argon, as shown in FIG. 1C; (ii) acarbon-containing liquid (e.g. acetone, methanol and ethanol) was mixedwith the argon stream by a bubbler (a similar procedure to water); and(iii) a solid (e.g. graphite and carbon nanotubes) were mixed with metalpowder.

For example, carbon-doped SnO₂ was prepared by introducing ethylene gas(C₂H₄) into flowing argon (200 sccm) for 2-4 hrs with the furnace at900° C. Carbon-doped WO₃ was obtained from 100 sccm Ar with 2 sccm C₂H₄at 760° C. for 1-4 hrs. The argon was also bubbled through water at 80°C. Carbon-doped TiOx was prepared by introducing both water and 30volume % acetone as a carbon source with 100-200 sccm Ar at 800-900° C.for 2-4 hrs.

Characterization of Nanostructures

The metal oxide nanostructures produced were characterized using atransmission electron microscope (TEM), high-resolution TEM (HRTEM),with electron energy-loss spectroscopy (EELS), energy dispersed X-rayspectroscopy (EDX), as well as with a scanning electron microscope (SEM)and field-emission scanning electron microscope (FE-SEM) with EDX.

Results:

One-dimensional nanostructures of three kinds of metal oxides (SnO,SnO₂, WO₃ and TiO_(x)), non-doped and doped, were synthesized by thevapor deposition methods described above. The detailed results will bepresented according to the following order (i) SnOx, (ii) WOx and (iii)TiOx.

Tin oxide nanostructures with varying amounts of oxygen were prepared.These materials were identified as SnO and SnO₂.

(1) SnO and SnO₂ nanostructures were grown on carbon paper and Al₂O₃ceramic substrates. FIG. 2A is a SEM image of commercially-used and barecarbon paper substrate. The carbon paper is widely used as an electrodematerial for electrochemical applications such as fuel cell backing andsensors. The carbon paper consists of small carbon fibers of 5-10 μm indiameter and a small piece of the paper was placed on the ceramic(alumina) boat overlying the tin powder in the boat. SnO and SnO₂nanostructures were first synthesized on carbon fibers of the carbonpaper substrate at 900° C. under a flow of oxygen-containing argon gasat a flow rate of 200 sccm for 2 hrs.

FIG. 2B portrays SnO, SnO₂ nanowires, grown on carbon fibers of carbonpaper substrate. The SEM image reveals a high density of the tin-oxygennanowires that cover the whole surface of carbon fibers. A TEM imageinset into a corner of FIG. 2B shows wire-like particles of nanoscaletin oxide particles. Obviously, such a nanostructure will have anappreciable and useful specific surface area.

The morphology of nanostructures can be controlled by changing thedistance of sample substrates from powder or molten metal sources.Generally, most growth areas close to the molten Sn source grewwire-like nanostructures. However, some areas more removed from the Snliquid produced needle-like (pointed ends) nanostructures on carbonpaper, as shown in FIG. 2C. In some areas, the second generation ofnanoneedles grown on top of the first generation of nanowires in aone-step synthesis process was also observed (FIG. 2D). Further, specialorientations between the first and the second generation of nanowireswere observed. The needle-like and second generation structures shouldbe associated with role of carbon from carbon paper and relative highergrowth temperature.

When the ceramic boat alone provided the support for the oxidized tin,it was found that the oxygen-containing nanoparticles formed asnanobelts—very thin and 10 μm wide—as illustrated in FIG. 2E.

(2). Carbon-Doped Tin and Carbon Composite Nanostructures

To improve electrical properties and corrosion resistance of SnOxnanostructures as well as to understand the role of carbon during growthof nanostructures, an experiment was conducted to produce in-situcarbon-doped SnOx structures by introducing ethylene gas (C₂H₄) in theflowing argon. But composites of tin nanowires in carbon nanotubes wereobtained. This is a new finding.

While keeping the same vapor deposition conditions as described in theabove section (1), 900° C. and 200 sccm argon flow rate, differentamounts (0.5 sccm-12 sccm) of carbon were introduced with the argon gasflow. The results showed that the different amounts of carbon resultedin different morphologies and structure of tin and carbon nanostructureson carbon paper as shown in FIGS. 3A through 3G.

FIGS. 3 (A-G) show SEM images of carbon-doped tin nanostructures oncarbon paper synthesized in 900 C, 200 sccm Ar and carbon amounts(0.5-12 sccm). In the case of 0.5 sccm ethylene (FIG. 3A), tin andcarbon nanostructures grown on a carbon fiber are not very dense and areabout 15 μm in length. With the increase of carbon amounts (1-5 sccm) inthe argon, the density of tin and carbon nanostructures significantlyincrease and totally cover the carbon fibers (FIGS. 3B-3D). But thelength is still about 15 μm. When the carbon compound flow rates in theargon flow reach 7 and 10 sccm, very short nanoparticles (1-5 μm) wereobtained (FIGS. 3E and 3F). For 12 sccm carbon compound flow rate, therewas no significant change in the structure of the particles; only morespherical nanostructures were observed (FIG. 3G).

A change of nanostructure morphologies is associated with structuralfeatures and composition of carbon-doped SnOx. The detailed analysiswill be illustrated in FIGS. 4A through 4F. Basically, carbon graphiticlayers were formed on the surface of the intended SnOx nanostructuresand metallic Sn nanostructures were obtained. When a sufficient amountof carbon was introduced into the argon gas, the growth of a Snnanostructure was promoted. When the amount of carbon is too high (12sccm), carbon limits the growth of the nanostructures. From theseresults it was recognized that a new nanostructure was obtained whencarbon amounts are in a range of 2-5 sccm. Details of these tin andcarbon composite structures will be presented in a further descriptionof FIGS. 4A-4F which is presented in the following section of thisspecification.

(3). Single-Crystalline Tin Nanowires Encapsulated by Carbon NanotubesGrown on a Substrate of Carbon Fibers

As mentioned above, a high-density of nanostructure wires was obtainedon a carbon fiber substrate when ethylene at flow rates of 2-5 sccm wasintroduced with the flow of argon. The SEM images of FIGS. 4A and 4Breveal their morphologies. Individual tin fibers are seen in FIG. 4Awhile the tin fibers are clustered like petals of a flower in some localareas as illustrated in FIG. 4B. However, close observations from SEMand HRTEM (FIGS. 4C and 4D) showed that the nanostructure actuallyconsists of carbon nanotubes (about 30 nm thick) at the tip and bottomof the structures, with tin nanowires in the middle of the fiber-likenanostructures. TEM images in FIGS. 4E and 4F gave more detailedinformation. In some cases, a carbon nanotube portion (white regionlabeled as C nanotube in FIG. 4E) appears between nanowire portions,labeled as Sn nanowire. In other cases, hollow nanotubes were found onlyin the tip of nanostructures (FIG. 4F). The compositional analysis byEDX (inset in FIG. 4E) showed that darker area of the nanowire iscomposed of tin covered by carbon layers. The tubular portion of thefiber is composed mostly of carbon.

2. Synthesis of WO₃ Nanostructures

As discussed in the section 4 (below), the growth mechanisms of WO₃nanostructures are different from the mechanisms for SnO, and SnO₂nanostructures. The growth of SnOx nanostructures follows vaporizationof Sn powder followed by oxidation of the tin and condensation of thenon-stoichiometric oxide on a substrate like carbon paper. The mechanismis a vapor-to-solid (VS) mechanism. In contrast, WO₃ (and TiOx)nanostructures appear to grow directly on W (or Ti) powders. Adifference in synthesis of WOx nanostructure is that the water-assistedprocess oxidation process was used. In this case, the argon flow, priorto entering the chamber, was bubbled through a hot water bath (80° C.)so that H₂O vapor was continuously transferred into the reaction zone.The introduction of water serves to provide more oxygen (maybe H₂ aswell) during the reaction. The control of water amount was carried outby water bubbling. The water-assisted process is very effective for thegrowth of WO_(x) and TiO_(x) nanostructures. FIGS. 5A-5F show SEM imagesof WOx, sulfur-doped WOx and carbon-doped WOx nanostructures.

FIGS. 5A (4,000×) and 5B (12,000×) show high-density clusters of WO₃monofilaments or nanowires formed on and covering underlying tungstenpowder. The WO₃ nanostructures are about 3-50 micrometers in length andabout 100 nm in diameter. When a small amount of sulfur (molar ratio ofW/S 3:1) was introduced in form of powder upstream of the tungstenpowder boat (as shown in FIG. 1B), a significant increase of density anduniform has been obtained as illustrated in the SEM images of FIGS. 5Cand 5D. For carbon-doping, a small amount of ethylene (2 sccm) isintroduced with wet Ar. The carbon-doped WOx nanostructures have similarmorphologies as WO₃ and sulfur-doped WO₃ as shown in FIGS. 5E and 5F.However, HRTEM images as presented in FIG. 6 showed that carbon-dopedWOx nanostructures were covered a layer of amorphous carbon.

3. Synthesis of TiOx Nanostructures

TiOx nanostructures were synthesized on Ti powder by chemical vapordeposition under conditions of 800-900° C. and Ar flow of 100-200 sccm.The results are illustrated in the SEM images of FIGS. 7A-7F.

FIG. 7A shows the morphology of the original Ti powder consisting ofrelatively large, generally spherical particles (10-15 micrometers indiameter). FIG. 7B is an SEM image of TiOx nanstructures synthesized andgrown on the titanium powder particles under an oxygen-containing argonstream. It can be seen that the surfaces of the titanium particles arecoarse and that they were oxidized into many polycrystalline grains.Both microwire and microbelt structures were found, although both ofthem are very sparse.

The process for forming TiOx particles on titanium powder was repeatedusing humidified argon. An argon stream was bubbled through a column ofwater at a rate of 30-80 bubbles per minutes. The humidified argonstream was then introduced into the quartz tube under the abovespecified conditions. As illustrated in the SEM images of FIGS. 7C (at1000×) and 7D (5000×) very dense nanoneedles were obtained. The TiOxnanoneedles had sharp and long tips (FIG. 7D).

When the argon stream contained acetylene, TiOx nanostructures having anew feature (called nanowall) appeared as shown in FIGS. 7E and 7F. Thenanowall structures were obtained by introducing into the flowing argonboth water and 30 vol. % acetone as a carbon source at 800-900° C. for2-4 hrs.

4. General Discussion of Nanostructure Growth Mechanisms

The growth mechanisms of SnOx nanostructures are different from themechanisms for WOx and TiOx nanostructures.

The SnOx nanostructure growth is governed by the vapor-solid (VS)mechanism. During the heating, Sn vapor is generated from the molten Snthan combines with oxygen, which comes from three sources: residualoxygen in the reaction chamber, oxygen “impurity” in the Ar gas, andsurface oxygen layers on metal powders. As a first step, Sn vapor andoxygen form SnO vapor. It is well-known that SnO is metastable and willdecompose into SnO₂ and Sn. The decomposition of SnO will result in theprecipitation of SnO₂ nanoparticles, which are carried by the flowing Argas and deposited on the walls of the alumina boat or carbon paper. Thenanoparticles then act as nucleation sites and initiate the growth ofSnO₂ nanostructures via the VS mechanism. When SnO₂ was formed on carbonpaper, the mechanism is more complicated due to the presence of carbonon the paper surface. The reaction of carbon with oxygen in the hotenvironment may reduce supply of oxygen and, eventually, produce Sn orSnO.

The growth mechanisms of WOx and TiOx grown on their powders are notclearly understood. At present it is believed that the presence of waterin the inert gas stream is necessary to obtain either WOx or TiOxnanostructures. The titanium and tungsten powders have higher meltingpoints than tin, and the titanium and tungsten powder produces lessvapor at the operating temperatures (700° C. to 1000° C.) of thereactor. The limited vapor production may account for the formation oftheir respective oxide nanostructures directly on their powders.

The growth of the composites of carbon nanotubes and tin nanowires isunderstood as follows. When heated at 900° C. under argon gas flow, theliquid tin (mp 232° C., by 2270° C.) produces vapor transporteddownstream by the argon and deposited as growing liquid droplets on thegraphitic fiber substrate. Two different nanostructure growth mechanismsoccur simultaneously at the substrate-borne tin droplets. Ethylene vapordecomposes on the tin droplets and forms carbon nanotubes while thecontinued condensation of tin provides for the later formation of tinnanowires. When the reactor is cooled, the tin solidifies within thecarbon nanotubes and shrinks to form the interesting carbon and tincomposite nanostructures.

The practice of the invention has been illustrated by several specificexamples but the scope of the invention is not limited by suchillustrations.

1. A method of making a composite of carbon nanotubes and tin nanowirescomprising: heating tin powder in a chamber to a temperature for formingtin vapor and condensing tin vapor as a liquid on a substrate whileflowing a mixture of an inert gas with a carbon-containing gas intocontact with the liquid tin to form carbon nanotubes in the liquid tin;and, after a predetermined time cooling the chamber to solidify the tinliquid as a composite of tin nanowires and carbon nanotubes, the tinnanowires being at least partially coated with carbon nanotubes.
 2. Amethod of making a composite of carbon nanotubes and tin nanowires asrecited in claim 1 in which the substrate comprises carbon fibers.
 3. Amethod of making a composite of carbon nanotubes and tin nanowires asrecited in claim 1 in which the tin liquid forms as droplets of tin onthe substrate.
 4. A method of making a composite of carbon nanotubes andtin nanowires as recited in claim 1 in which the temperature in thechamber is in the range of about 700° C. to about 1000° C.
 5. A methodof making a composite of carbon nanotubes and tin nanowires as recitedin claim 1 in which the inert gas is argon.
 6. A method of making acomposite of carbon nanotubes and tin nanowires as recited in claim 1 inwhich the carbon-containing gas is ethylene.
 7. A method of making acomposite of carbon nanotubes and tin nanowires as recited in claim 1 inwhich the flowing mixture comprises argon and ethylene.
 8. A method ofmaking a composite of carbon nanotubes and tin nanowires as recited inclaim 1 in which the flowing mixture comprises argon and ethylene, theethylene being present in the mixture in a concentration of less than 6%by volume.
 9. A method of making a composite of carbon nanotubes and tinnanowires as recited in claim 8 in which the ethylene is present in aconcentration of between 1 and 2.5% by volume.
 10. A composite of carbonnanotubes and tin nanowires formed by the method recited in claim
 1. 11.A composite of carbon nanotubes and tin nanowires formed by the methodrecited in claim
 7. 12. A composite of one-dimensional nanostructurescomprising tin nanowires at least partially contained in carbonnanotubes.